Java Array and ArrayList

Array key point:

• Not scalable, once constructed length cannot be change,
• When construct numeric primitive type, it will filled with default value. byte, short, int, long, double, float 0
• char filled with empty.
• boolean filled with false.
• object filled with null.

public static void main(String[] args) {
  byte[] bytesArray = new byte[1];
  System.out.println(bytesArray[0]);
		
  short[] shortArray = new short[1];
  System.out.println(shortArray[0]);
		
  char[] charArray = new char[1];
  System.out.println(charArray[0]);
		
  int[] intArray = new int[1];
  System.out.println(intArray[0]);
		
  long[] longArray = new long[1];
  System.out.println(longArray[0]);
		
  float[] floatArray = new float[1];
  System.out.println(floatArray[0]);
		
  double[] doubleArray = new double[1];
  System.out.println(doubleArray[0]);
}

## Output ##
0
0

0
0
0.0
0.0

Copy by reference

When copy array using equal assignment it will point to the origin reference. Below example shown both array1 and array2 elements are same.

int[] array1 = { 1, 2, 3, 4, 5 };
int[] array2 = array1;
array2[4] = 7;
		
System.out.println(Arrays.toString(array1));
System.out.println(Arrays.toString(array2));

## Output ##
[1, 2, 3, 4, 7]
[1, 2, 3, 4, 7]

Copy by value

To copy one from another without interfere it’s original value, use Arrays.copyOf.

int[] array1 = { 1, 2, 3, 4, 5 };
int[] newarray = Arrays.copyOf(array1, array1.length);
newarray[4] = 7;

System.out.println(Arrays.toString(array1));
System.out.println(Arrays.toString(newarray));

## Output ##
[1, 2, 3, 4, 5]
[1, 2, 3, 4, 7]

Arrays.fill

Arrays class provides methods that let you fill/replace all elements with additionally supply with start and end index.

String[] transport = {"car", "bike", "van", "lori"};
Arrays.fill(transport, 1, 2, "bus");
		
System.out.println(Arrays.toString(transport));
## Output ##
[car, bus, van, lori]

Arrays.equals

To compare 2 array elements equality, we can use Arrays.equals.

int a[] = { 1, 2, 3, 4};
int b[] = { 2 ,4 ,6 ,8};
int c[] = { 1, 2, 3 ,4};

System.out.println(Arrays.equals(a, b)); // false
System.out.println(Arrays.equals(a, c)); // true
System.out.println(Arrays.equals(b, c)); // false

ArrayList

ArrayList key point:

• Only support primitive wrapper and object.
• Scalable, create new array internally when insufficient size.
• ArrayList is a generic class

ArrayList<Integer> numbers = new ArrayList<>();

Copy by Reference

ArrayList work as same when assigned to new variable. One changed will affected anther to their origin values.

ArrayList<Integer> digits = new ArrayList<>();
digits.add(1);
digits.add(2);
digits.add(3);
		
ArrayList<Integer> newDigit = digits;
newDigit.set(0, 10);
newDigit.set(1, 20);
newDigit.set(2, 30);
		
System.out.println(digits.toString());
System.out.println(newDigit.toString());

## Output ##
[10, 20, 30]
[10, 20, 30]

Copy by value

To copy by it’s value, use constructor parameters.

ArrayList<Integer> digits = new ArrayList<>();
digits.add(1);
digits.add(2);
digits.add(3);
		
ArrayList<Integer> newDigit = new ArrayList<>(digits);
newDigit.set(0, 10);
newDigit.set(1, 20);
newDigit.set(2, 30);
		
System.out.println(digits.toString());
System.out.println(newDigit.toString());

## Output ##
[1, 2, 3]
[10, 20, 30]

Collection.fill

Similar to Arrays.fill, ArrayList can be fill by using Collection.fill but it cannot specify with start and end index.

String[] transport = {"car", "bike", "van", "lori"};
List<String> list = Arrays.asList(transport);

Collections.fill(list, "unknown");
System.out.println(list);

Array and ArrayList as Parameter

When Array or ArrayList pass as parameter, we refer to pass as reference. Which mean the method received a reference to the the array.

public static void main(String[] args) {
  int[] param = {1, 2, 3, 4, 5, 6};
  System.out.println("Orignal " + Arrays.toString(param));
		
  refByValue(param);
  System.out.println("Param" + Arrays.toString(param));

  param = test1(param);
  System.out.println("Test 1: " + Arrays.toString(param));
		
  test2(param);
  System.out.println("Test 2j: " + Arrays.toString(param));
}
	
public static int[] test1(int[] param) {
  int[] newParam = param;
  Arrays.fill(newParam, 0);
  return newParam;
}
	
public static void test2(int[] param) {
  Arrays.fill(param, 100);
}

public static void refByValue(int[] param) {
  int[] arr = Arrays.copyOf(param, param.length);
  Arrays.fill(arr, 0);
  System.out.println("newParam: " + Arrays.toString(arr));
}

## Output ##
Orignal [1, 2, 3, 4, 5, 6]
newParam: [0, 0, 0, 0, 0, 0]
Param[1, 2, 3, 4, 5, 6]
Test 1: [0, 0, 0, 0, 0, 0]
Test 2j: [100, 100, 100, 100, 100, 100]

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Java Array, ArrayList Conversion